同济大学：《工程热力学》课程电子教案（讲稿）Chapter 2 Energy First Law of Thermodynamics

2013-3-1 Learning Outcomes Demonstrate understanding of key concepts related to energy and the first law of thermodynamics...including internal,kinetic and potential energy,work and power,heat transfer and heat transfer modes,heat transfe rate,power cycle,refrigeration cycle,and heat pump cycle. 23 Learning Outcomes,cont. Apply closed system energy balances appropriately modeling the case at hand,and correctly observing sign conventions for work and heat transfer. Conduct energy analyses of systems undergoing thermodynamic cycles,evaluating as appropriate thermal efficiencies of power cycles and coefficients of performance of refrigeration and heat pump cycles. 24 2

2013-3-1 2 Learning Outcomes ►Demonstrate understanding of key concepts related to energy and the first law of thermodynamics. . . including internal, kinetic, and potential energy, work and power, heat transfer and heat transfer modes, heat transfer rate, power cycle, refrigeration cycle, and heat pump cycle. 2-3 Learning Outcomes, cont. ►Apply closed system energy balances, appropriately modeling the case at hand, and correctly observing sign conventions for work and heat transfer. ►Conduct energy analyses of systems undergoing thermodynamic cycles, evaluating as appropriate thermal efficiencies of power cycles and coefficients of performance of refrigeration and heat pump cycles. 2-4

2013-3-1 Introduction Q:Refrigerator open in a well-sealed and well-insulated room.Will the temperature increase,stay the same,or go down? A:Considering the room as the system(adiabatic)the only Room energy consumed by the device to heat,the room temperature will rise. Energy cannot be created or destroyed A refrigerator operating with its door open in a well-sealed and well- insulated room 2-5 Review:Forms of Energy >Energy exists in numerous forms:thermal mechanical,kinetic,potential,electric, magnetic,chemical,and nuclear,and their sum constitutes the total energy,E of a system. sony vin the change >Macroscopic forms of energy:e.g.,kinetic and potential energies. Microscopic forms of energy:relate ed to the ofe molecl structure of a system and the degree mofnerfe 2-6 3

2013-3-1 3 A refrigerator operating with its door open in a well-sealed and well￾insulated room Introduction Q: Refrigerator open in a well-sealed and well-insulated room. Will the temperature increase, stay the same, or go down? A: Considering the room as the system (adiabatic) the only energy interaction involved is the electrical energy crossing the system boundary and entering the room. As a result of the conversion of electric energy consumed by the device to heat, the room temperature will rise. 2-5 Energy cannot be created or destroyed Review: Forms of Energy ¾ Energy exists in numerous forms: thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy, E of a system. ¾ Thermodynamics deals only with the change of the total energy. ¾ Macroscopic forms of energy: e.g., kinetic and potential energies. ¾ Microscopic forms of energy: related to the molecular structure of a system and the degree of the molecular activity. 9 Internal energy, U: The sum of all the microscopic forms of energy. 2-6

2013-3-1 Review:Mechanical Concepts of Energy >Kinetic energy,KE:The energy that a system possesses as a result of its motion relative to some reference frame. >Potential energy,PE: The energy that a system possesses as a result of its elevation in a gravitational field >Both are extensive properties of the body Earth's surf 2-7 Change in Kinetic Energy The change in kinetic energy is associated with the motion of the system as a whole relative to an external coordinate frame such as the surface of the earth. For a system of mass m the change in kinetic energy from state 1 to state 2 is AKE=KE2-KE=mV2-VP)(Eq.2.5) where -V and V,are the initial and final velocity magnitudes -The symbol A denotes:final value minus initial value. 2-8

2013-3-1 4 ¾ Kinetic energy, KE: The energy that a system possesses as a result of its motion relative to some reference frame. ¾ Potential energy, PE: The energy that a system possesses as a result of its elevation in a gravitational field. ¾ Both are extensive properties of the body 2-7 Review: Mechanical Concepts of Energy where - V1 and V2 are the initial and final velocity magnitudes - The symbol Δ denotes: final value minus initial value. Change in Kinetic Energy ►The change in kinetic energy is associated with the motion of the system as a whole relative to an external coordinate frame such as the surface of the earth. ►For a system of mass m the change in kinetic energy from state 1 to state 2 is 2-8 ΔKE = KE2 – KE1 = ( ) 2 1 2 V2 V 2 1 m − (Eq. 2.5)

2013-3-1 Change in Gravitational Potential Energy The change in gravitational potential energy is associated with the position of the system in the earth's gravitational field. For a system of mass m the change in potential energy from state 1 to state 2 is APE=PE2-PE1=mg(2-1) where -z,and z,denote the initial and final elevations relative to the surface of the earth,respectively. g is the acceleration of gravity. 2-9 Conservation of Energy in Mechanics △PE+△KE=0 R=0 2m(v号-vW+mg(a-)=0 Consider only g 2mV号+mg2=2mv+mg KE2 +PE2=KE +PE =constant Sum of kinetic and potential energies remain constant 2.10 5

2013-3-1 5 Change in Gravitational Potential Energy ►The change in gravitational potential energy is associated with the position of the system in the earth’s gravitational field. ►For a system of mass m the change in potential energy from state 1 to state 2 is 2-9 ΔPE = PE2 – PE1 = mg(z2 – z1) where - z1 and z2 denote the initial and final elevations relative to the surface of the earth, respectively. - g is the acceleration of gravity. Conservation of Energy in Mechanics 2-10 ΔPE + ΔKE = 0 Sum of kinetic and potential energies remain constant KE2 + PE2 = KE1 + PE1 =constant R=0 Consider only g

2013-3-1 Change in Internal Energy The change in intemal energy is associated with the makeup of the system,including its chemical composition. There is no simple expression for evaluating internal energy change for a wide range of applications.In most cases we will evaluate internal energy change using data from tables in appendices of the textbook Like kinetic and gravitational potential energy, internal energy is an extensive property. Internal energy is represented by U. The specific internal energy on a mass basis is u The specific internal energy on a molar basis is a 2-11 Change in Energy of a System In engineering thermodynamics the change in energy of a system is composed of three macroscopic contributions: Kinetic energy Gravitational potential energy Internal energy(all other energy changes are lumped together) Total energy and internal energy are also extensive properties. 2-12 6

2013-3-1 6 Change in Internal Energy u. 2-11 ►The change in internal energy is associated with the makeup of the system, including its chemical composition. ►There is no simple expression for evaluating internal energy change for a wide range of applications. In most cases we will evaluate internal energy change using data from tables in appendices of the textbook. ►Like kinetic and gravitational potential energy, internal energy is an extensive property. ►Internal energy is represented by U. ►The specific internal energy on a mass basis is u ►The specific internal energy on a molar basis is ū Change in Energy of a System 2-12 In engineering thermodynamics the change in energy of a system is composed of three macroscopic contributions: ►Kinetic energy ►Gravitational potential energy ►Internal energy (all other energy changes are lumped together) Total energy and internal energy are also extensive properties

2013-3-1 Review:Mechanical Concepts of Energy v2 Kinetic energy KE=m2 (kJ) Potential energy PE =mg (kJ) Total energy of a system v2 E=U+KE+PE-U+m+ (kJ) Energy of a system per unit mass e=u+ke+pe-u+2 (kJ/kg） 2-13 Units for Energy Units for kinetic energy,potential energy and work are the same and they are: SI:N-m=Jor kJ (Joule) English:foot-pound force ft-lbfor British Thermal Unit,BTU 2-14 7

2013-3-1 7 Review: Mechanical Concepts of Energy 2-13 Total energy of a system Energy of a system per unit mass Potential energy Kinetic energy Units for kinetic energy, potential energy and work are the same and they are: SI: N·m = J or kJ (Joule) English: foot-pound force ft·lbf or British Thermal Unit, BTU Units for Energy 2-14

2013-3-1 0 Total Energy 2-15 Summary:Energy of a System In summary,the change in energy of a system from state 1 to state 2 is E2-E,=(U2-U)+(KE2-KE)+(PE2-PE) △E=△U+△KE+△PE Since an arbitrary value E can be assigned to the energy of a system at a given state 1,no particular significance can be attached to the value of energy at state 1 or any other state.Only changes in the energy of a system between states have significance. 2-16 8

2013-3-1 8 Total Energy 2-15 http://bcs.wiley.com/he-bcs/Books?action=index&itemId=0470918012&bcsId=6606 Summary: Energy of a System 2-16 ►In summary, the change in energy of a system from state 1 to state 2 is E2 – E1 = (U2 – U1) + (KE2 – KE1) + (PE2 – PE1) ΔE = ΔU + ΔKE + ΔPE ►Since an arbitrary value E1 can be assigned to the energy of a system at a given state 1, no particular significance can be attached to the value of energy at state 1 or any other state. Only changes in the energy of a system between states have significance

2013-3-1 Energy Transfer by Work Heat System boundary Energy can be transferred to and from closed systems CLOSED by two means only: SYSTEM Work Work (m=constant) Heat You've most probably studied work in mechanics. However,thermodynamics deals with phenomena not included within the scope of mechanics,and this requires a broader interpretation of work. 2-17 Heat vs.Work >Both are recognized at the boundaries of a 8g8tenaneycostnebounares6ouincen Systems possess energy,but not heat or work. >Both are associated with a process,not a state Unlike properties,heat or work has no meaning at a state. Both are path functions(i.e..their magnitudes depend on the path followed during a process as well as the end states). >Both are directional quantities:have magnitude AND direction 9

2013-3-1 9 Energy Transfer by Work & Heat ►Energy can be transferred to and from closed systems by two means only: ►Work ►Heat ►You’ve most probably studied work in mechanics. However, thermodynamics deals with phenomena not included within the scope of mechanics, and this requires a broader interpretation of work. 2-17 Heat vs. Work ¾ Both are recognized at the boundaries of a system as they cross the boundaries (boundary phenomena) ¾ Systems possess energy, but not heat or work. ¾ Both are associated with a process, not a state. ¾ Unlike properties, heat or work has no meaning at a state. ¾ Both are path functions (i.e., their magnitudes depend on the path followed during a process as well as the end states). ¾ Both are directional quantities: have magnitude AND direction

2013-3-1 Heat vs.Work Properties are point functions/have exact differentials(d). (2 V=-V=△V J Path functions(Heat and work)have inexact differentials(8) 2m 2 Properties are point functions; 8W=W12 (not△W） but heat and work are path functions(their magnitudes depend on the path followed). 2.19 Illustrations of Work When a spring is compressed, energy is transferred to the spring by work. When a gas in a closed vessel is stirred,energy is transferred to the gas by work. When a battery is charged electrically,energy is transferred to the battery contents by work. The first two examples of work are encompassed by mechanics.The third example is an example of the broader interpretation of work encountered in thermodynamics. 2-20 10

2013-3-1 10 Heat vs. Work 2-19 Properties are point functions; but heat and work are path functions (their magnitudes depend on the path followed). Properties are point functions / have exact differentials (d ). Path functions (Heat and work) have inexact differentials (δ ) ►When a spring is compressed, energy is transferred to the spring by work. ►When a gas in a closed vessel is stirred, energy is transferred to the gas by work. ►When a battery is charged electrically, energy is transferred to the battery contents by work. Illustrations of Work ►The first two examples of work are encompassed by mechanics. The third example is an example of the broader interpretation of work encountered in thermodynamics. 2-20

2013-3-1 Mechanical Forms of Work Two requirements for a work interaction between a system and its surroundings to exist: -there must be a force acting on the boundary -the boundary must move. zz） When force is not constar Nork=Forcex Distanc W=F() (kJ If there is no movement no work is done© Expansion or Compression Work A月 Moving boundary work SW=pAdx SW =pdv W=「pdV Work is not a property! pv"=constant Polytropic process 2-22 11

2013-3-1 11 Mechanical Forms of Work Two requirements for a work interaction between a system and its surroundings to exist: – there must be a force acting on the boundary. – the boundary must move. The work done is proportional to the force applied (F) and the distance traveled (s). Work = Force × Distance When force is not constant If there is no movement, no work is done ☺ Expansion or Compression Work 2-22 2 1 d d d V V W pA x W pV W pV δ δ = = = ∫ constant n pv = Work is not a property! Moving boundary work Polytropic process